Various types of switching networks are known in the art. Some of these types have hierarchical topologies, meaning that the switches in the network are connected together in multiple stages (or levels). A signal input at one of the edge ports of the switching network, at the bottom of the hierarchy, must pass through one or more internal stages, at higher levels in the hierarchy, before reaching another edge port from which it is output. Some hierarchical networks are non-blocking, in the sense that any unused edge port can always be connected to any other unused edge port, regardless of existing connections within the network.
A Clos network is a kind of hierarchical switching network, which is also referred to as a CBB or Fat-Tree network. This type of network topology was first formalized by Charles Clos in “A Study of Non-Blocking Switching Networks,” Bell System Technical Journal 32 (1953), pages 406-424. A Clos network is made up of crossbar switches arranged in interconnected stages, wherein the number of ports on each individual switch is typically much smaller than the total number of edge ports of the network. Although Clos networks were originally conceived for use in public switched telephone networks, they have gained popularity in some packet switching data applications. A review of some applications of this latter sort is provided in a white paper entitled “Scaling 10 Gb/s Clustering at Wire-Speed,” published by Mellanox Technologies Inc. (Santa Clara, Calif., 2006), which is incorporated herein by reference.
The Spanning Tree Protocol (STP), defined in IEEE Standard 802.1D, is a network protocol that may be used to ensure a loop-free topology in any bridged local area network (LAN). As the protocol runs over the nodes of a network, it creates a tree of connections among the nodes, while disabling links that are not part of the tree, so that there is only a single active path between any two nodes. The above-mentioned white paper points out that the use of STP in a Clos network may cause problems of lost bandwidth and congestion.